Light emitting diode (LED) lighting systems are becoming more prevalent as replacements for existing lighting systems. LEDs are an example of solid state lighting (SSL) and have advantages over traditional lighting solutions such as incandescent and fluorescent lighting because they use less energy, are more durable, operate longer, can be combined in red-blue-green arrays that can be controlled to deliver virtually any color light, and contain no lead or mercury. In many applications, one or more LED dies (or chips) are mounted within an LED package or on an LED module, which may make up part of a lighting unit, lamp, “light bulb” or more simply a “bulb,” which includes one or more power supplies to power the LEDs. An LED bulb may be made with a form factor that allows it to replace a standard threaded incandescent bulb, or any of various types of fluorescent lamps. LEDs can also be used in place of florescent lights as backlights for displays.
LEDs can be made of various materials, including silicon carbide and sapphire. Organic materials can also be used to make LEDs. A so-called “organic LED” (OLED) is made by placing a series of organic thin films between two conductors. When electrical current is applied, it passes from a negatively charged (cathode) film layer to a positively charged (anode) film layer, and the energy from the electrical current stimulates organic material, which in turn emits light visible through an outermost layer of the OLED structure.
Color reproduction can be an important characteristic of any type of artificial lighting, including LED lighting. For lamps, color reproduction is typically measured using the color rendering index (CRI). The CRI is a relative measurement of how the color rendition of an illumination system compares to that of a particular known source of light. In more practical terms, the CRI is a relative measure of the shift in surface color of an object when lit by a particular lamp. The CRI equals 100 if the color coordinates of a set of test surfaces being illuminated by the lamp are the same as the coordinates of the same test surfaces being irradiated by the known source. CRI is a standard for a given type light or light from a specified type of source with a given color temperature. A higher CRI is desirable for any type of replacement lamp.